Advanced semi-conductor circuitry may use various programmable interconnect elements to connect logic blocks for a number of applications, such as electrically programmable feature selection, speed sorting, die identification, and redundancy implementation. For high-speed applications, the on-state of such structures must have a low resistance. To achieve a high density of such structures in an integrated circuit, the programmable elements should be small.
Programmable fuses are an example of a programmable element. Programmable fuses fall into at least three categories depending on the method of programming. The first category includes laser-blown fuses where the programmable fuses are programmed using a laser to burn or sever the conductive portion of the fuse. The second category includes electrically blown fuses where the fuse is programmed by passing current through it sufficient to overload and open or burn out the fuse. The third category includes electrically blown anti-fuses, where the fuse is programmed with an electric current, which reduces the resistance across the fuse.
In all of these methods of fuse programming, the area surrounding the fuse may be damaged during the programming process due to the heat involved in the programming step, and thus the fuses must have sufficient space between one another on a wafer so that the programming of one fuse does not damage adjacent fuses. For example, laser fusing produces damage to the area surrounding the fused element, which is traded off for fuse blow yield. Another problem with laser fusing is that it requires a large on-chip area to handle laser power capability as well as “line-of-sight” for laser access (which is a component of the area penalty of the laser fusing process). Excessive laser energy can cause silicon substrate damage or massive crater formation that impacts neighboring links. To further aggravate the problem, the laser fuse programming requires that all customization and repair data be collected, and stored off-line after each test. Once all data has been collected, it must be compiled into a single repair solution and translated into XY coordinates corresponding to the fuse locations on the chip. Such problems with laser fusing are not necessarily mitigated by traditional electrically blown fuses because electrically blown fuses typically require a larger voltage than is conveniently available on the chip and their performance and reliability are still to be established.
The typical electrically blown fuse consists of a poly/silicide stack (e.g., CoSi2) at the gate level using a high-current induced electromigration mechanism to program the fuse. However, the typical electrically blown fuse has a relatively complicated structure which may include the polysilicon fuse, a fuse latch, a program latch, a program FET, and a look-ahead programming multiplexer. This complexity adds to the cost of such fuses. Furthermore, for the case of the electrically blown fuse, the post fuse resistance value may have a large variance due to various electromigration induced void sizes, thus making it difficult to precisely control the resistance of the blown fuse. Additionally, the deletion rate of a typical electrically blown fuse is less than 100%, meaning that in many instances after the fusing process, the fuse is not blown and thus not programmed, potentially leading to an inoperative circuit. Also noteworthy in the typical electrically blown fuse is that a blown fuse may heal itself or re-growth during subsequent operation of the surrounding circuitry. Also, such typical fuses are less than stable both in the programmed and unprogrammed state due to void healing under high temperature storage without electrical current.
Additionally, the on-state resistance of a typical electrically blown fuse can not be tightly or precisely controlled. Furthermore, the typical electrically blown fuse is best implemented in the front end of line of manufacturing rather than the back end of line of manufacturing, thus increasing manufacturing costs and reducing design flexibility due to being restricted to a poly-silicon layer. Also, typical electrically blown fuses suffer from a sensitivity change of sensing circuitry over time with device degradation.
BEOL electrically blown fuses using self Joule heating to achieve melting open have been reported. For example, Table 1 shows measured on-state resistance in Ohms (Ω), calculated melting current in milliamps (mA) and melting voltage (V) based on TaN melting temperature (3100° C.), for various traditional BEOL electrically blown TaN fuses of various noted dimensions. The on-state or pre-deletion resistance is the amount of resistance provided by the fuse before programming or deletion. The melting current is the amount of current required to program or delete the fuse, and melting voltage is the voltage required to program or delete the fuse. For example, the first row of Table 1 indicates that for a fuse ten microns wide and ten microns long, the on-state resistance is 56.59Ω, the calculated melting current is 126.28 mA, and the calculated melting voltage is 7.15 V based on TaN melting temperature equal to about 3100° C.
TABLE 1LengthOn-stateRequiredRequiredWidth (um)(um)ResistanceMelting CurrentMelting Voltage101056.59126.287.151020114126.2814.40201028.12208.305.86202057.01208.3011.881530114.5167.8919.221545172.3167.8928.931050287.1126.2836.261575287.6167.8948.28
As can be seen from Table 1, due to the extremely high melting temperature of TaN material, traditional BEOL TaN electrically blown fuses require a high melting current and high melting voltage with a concurrent potential to damage surrounding components during programming or deletion.